Population structuring by weirs and the effect on trophic position of a freshwater fish Zacco platypus in the middle reaches of Japanese rivers

Abstract

The barrier effect of weirs in a river course was investigated for freshwater fish populations in middle reaches of Japanese temperate rivers. For this purpose, the genetic and trophic aspects of populations of a freshwater fish Zacco platypus were compared between two river courses, one with weirs and the other without. Genetic distance, as a measure of the extent of genetic differences between two populations of organisms, was determined using microsatellite genetic markers. Trophic position, a continuous measure of the location of a population or a species within a food web, was determined using stable nitrogen and carbon isotopes. A significant genetic pattern of isolation by distance was observed among subpopulations for the river with weirs, but not for the river without weirs. It is concluded that population structuring resulted from the barrier effect of weirs on fish migration. Trophic positions of subpopulations of the fish declined upstream in both rivers. However, when changes in trophic position were scaled against the length of river channel between subpopulations, a significant pattern of position change by distance was observed among subpopulations for the river with weirs, but not for the river without weirs. The significant change in trophic position may have arisen from population structuring due to limited access to food resources or river habitats.